Portable direction finding systems are well-known in the prior art. One such system consists of a loop antenna with a deployable whip attached to the side of the loop. To determine the direction of a transmitter, the whip is first deployed to produce a coarse cardioid-shaped antenna pattern. The strength of the received signal is monitored while the antenna is rotated to produce a maximum signal. This first step indicates the general direction of the transmitter. Thereafter, the whip is retracted and one of two sharp nulls of the loop antenna is then used to accurately determine the transmitter direction. Such portable direction finding systems require substantial operator training and several minutes of time for a proper determination of the transmitter direction.
It is also known in the prior art to locate the direction of a transmitter using a three antenna system including first and second orthogonally-mounted loop antennas in combination with a omnidirectional antenna. Such systems typically use the electrical summation of all the antenna outputs to create composite antenna patterns. These systems are therefore undesirable because they require precise control of antenna placement and precise matching of the antenna amplitude and phase characteristics. Such requirements increase the overall cost of the system while at the same time decreasing reliability due to antenna and signal processing component mismatch.
Accordingly, there is therefore a need for an improved method and apparatus for deriving the direction of a remote transmitter which overcomes these and other problems associated with prior art direction finding systems.